© 2001 Nature Publishing Group http://structbio.nature.com news and views Two insulators are not better than one Fabien Mongelard and Victor G. Corces Chromatin insulators seem to control eukaryotic gene expression by regulating interactions between enhancers and promoters. Mounting evidence suggests that these sequences play a complex role in the regulation of transcription, perhaps mediated by changes in chromatin structure and nuclear organization. The expression of eukaryotic genes in spe- the largest collection of these sequences so structures, such as legs and antennae, are cific tissues and at particular times of far. The specialized chromatin structure changed into one another. These results development is determined by regulatory (scs and scs′ elements) flank one of the suggest that the Fab-7 insulator is required sequences called enhancers and silencers. heat shock protein 70 (hsp70) gene loci, for proper gene expression in order to Enhancers bind transcription factors, where they seem to delimit the region that avoid interference among the different reg- which then help initiate transcription of becomes transcriptionally active (or ulatory sequences of the Abd-B gene4–6. A the RNA at the promoter. Enhancers puffed) in response to heat shock. Other second example that illustrates the role of accomplish this independently of their Drosophila insulators include those found insulators in vivo is that of imprinting in distance and location with respect to the in the so-called gypsy retrovirus and the mice. Imprinting is an epigenetic trait gene. This flexibility in enhancer function Fab-7 and Fab-8 elements of the bithorax often seen as differences between paternal also gives these sequences the potential of complex, a complex containing three of and maternal inheritance. The parent-of- being highly promiscuous. Since this the Hox genes that encode important origin-specific expression of the mouse promiscuity is rarely observed, eukaryotes developmental control proteins in flies. H19 and Igf2 genes is controlled by an must have developed mechanisms to Insulators have also been studied in verte- insulator element located between the two insure that genes are not activated in the brates, where they have been found in the genes7,8. wrong tissue or at the wrong time of ribosomal RNA genes of Xenopus, the development by enhancers from a neigh- chicken β-globin genes and the human T Two possible models boring gene. Chromatin insulators may cell receptor (TCR)-α/δ locus among oth- Much of the work on chromatin insulators play the role of gatekeepers that curtail ers. The widespread distribution of insu- has centered on the characterization of enhancer promiscuity. lators is suggestive of an important role associated proteins with the goal of Boundaries or insulator elements have for these sequences in the biology of the explaining how these sequences affect © http://structbio.nature.com Group 2001 Nature Publishing been experimentally identified based on nucleus. enhancer function, however, progress in two defining characteristics. Insulators An idea of the role insulators could play the field has been hampered by the lack of interfere with interactions between in the cell can be gleaned from the analysis understanding of how enhancers activate enhancers and promoters and inhibit of their distribution in the bithorax com- transcription. Enhancers are currently enhancer-activated transcription, specifi- plex of Drosophila. The various genes of viewed as entry points for transcription cally when interposed between the affect- this complex are expressed in a paraseg- factors which, once bound, can interact ed enhancer and the promoter. Insulators ment-specific pattern (a parasegment is a with the transcription complex by looping also buffer transgenes, inserted in the metameric unit along the anterior-posteri- out intervening sequences9. More recently, genome at random sites, from chromoso- or axis of flies that is out of phase with seg- enhancer-bound transcription factors mal position effects1. Based on the first ments) by a complex set of regulatory have been shown tethered to proteins at property, insulators can be viewed as an sequences arranged in a linear fashion over the promoter region that are involved in additional class of regulatory sequences in 300 kb of DNA, corresponding to the order histone modification or other types of the arsenal of controlling elements at the of expression of the genes along the anteri- chromatin remodeling complexes10. Based service of genes to ensure their proper or-posterior axis of the animal. These on these findings, the effects of insulators spatial and temporal regulation. But the parasegment-specific regulatory sequences on enhancers could be explained by latter property seems to suggest a more appear to be separated by insulator ele- assuming that the insulator acts as a barri- complex role, perhaps involving the orga- ments, one of which is called Fab-7. The er that inhibits the processivity of a signal nization and compartmentalization of the Fab-7 insulator is located between two traveling from the enhancer to the pro- chromatin fiber within the nucleus. Two enhancers named iab-6 and iab-7 that con- moter, without inactivating either. The papers recently published in Science trol expression of the Abdominal-B leitmotiv of this scenario, sometimes appear to support this interpretation2,3. (Abd-B) gene in parasegments PS11 and dubbed the ‘transcriptional model’, is that PS12, respectively. Deletion of the Fab-7 the insulator acts as a promoter decoy, Properties of an insulator insulator eliminates the gatekeeper nor- confusing the enhancer-bound transcrip- Insulator elements have been described in mally ensuring that the iab-6 and iab-7 tion factor into interacting with the insu- many organisms, including yeast, enhancers activate transcription of the lator instead of the transcription Drosophila and vertebrates1. In yeast, insu- Abd-B gene in the appropriate tissues. complex11. This would result in the insula- lators appear to delimit the boundaries of Without the Fab-7 insulator, the iab-7 tor trapping the enhancer, which then silenced chromatin at telomeres and the enhancer activates expression of Abd-B in would not be able to interact with the pro- mating-type loci HML and HMR. the wrong cells, causing homeotic pheno- moter, thus precluding transcriptional Drosophila insulators were the first to be types in the adult fly. Homeotic pheno- activation. Another explanation, termed characterized and this organism displays types are those in which homologous the ‘structural model’ envisions insulators 192 nature structural biology • volume 8 number 3 • march 2001 © 2001 Nature Publishing Group http://structbio.nature.com news and views a the VRE enhancer is more efficiently blocked (Fig. 1c). Similar observations were made by Muravyova et al.3 in their companion paper. Using yellow and white as reporter genes, they show that a tandem repeat of b gypsy insulators does not have any insu- lating properties (Fig. 1b). Indeed, the authors note that enhancer–promoter communication in this configuration is improved. This holds true even when a c 1.5 kilo base pair spacer was present between the two insulators, ruling out the trivial possibility of some steric hindrance between insulators. These authors con- tribute an additional important result. d One of the constructs tested carried two copies of the gypsy insulator between the enhancers and the yellow gene, and an additional copy between yellow and white. In such a configuration both genes are Fig. 1 Effects of insulators on enhancer–promoter interactions. The diagrams represent a generic fully active (Fig. 1d). gene, depicted in blue, with a series of proteins forming the transcription complex present in the promoter region. Two enhancers, named Enhancer1 and Enhancer2, serve as binding sites for Role of insulators in chromatin transcription factors depicted in pink and purple, respectively. An insulator is shown in brown with two protein components represented as blue and green ovals. Green arrows indicate transcrip- domain organization tional activation by a specific enhancer. a, An insulator present between Enhancer1 and It is difficult to reconcile these observa- Enhancer2 affects the ability of the former to activate transcription (indicated by a red X) but not tions with a transcriptional insulator the latter. b, When an additional insulator is present between the two enhancers, the dual insula- tor cannot interfere with transcription activation by Enhancer1. c, The simple duplication of insu- model. For example, a reasonable pre- lators is not responsible for their loss of function, since when Enhancer1 is present between the diction from the decoy model is that two tandemly repeated insulator it is unable to activate transcription. d, In a more complex situation, a insulators would be better than one. second reporter gene is present downstream from the first one. Transcription of this gene can also That is, a tandem repeat of insulators be activated by Enhancer1. When tandemly repeated insulators are present and an additional insulator is present between the two genes, both genes are transcribed. would reinforce the trapping of the © http://structbio.nature.com Group 2001 Nature Publishing enhancer. Similarly, if insulators were entry points for chromatin modifying enzymatic complexes, the presence of as sequences that organize the chromatin This insulator has been shown to contain two insulators should result in a dou-